1. Field of the Invention
The present invention relates to a method for producing magnetic oxide materials which are high in surface activity and suitable for low temperature sintering.
2. Prior Art
Up to now, various methods have been proposed for producing magnetic oxide powders, for example, Ni--Zn ferrite powders. These methods includes (a) a powder-mixing method comprising the steps of mixing carbonates and/or oxides of respective constituents of a Ni--Zn ferrite in given proportions and calcining the resultant mixture at a high temperature to produce a Ni--Zn ferrite powder, (b) an oxalate method including the steps of incorporating an oxalate into a mixed aqueous solution of water-soluble compounds of Ni, Zn and Fe to precipitate their oxalate and calcining the resultant oxalate to produce a Ni--Zn ferrite powder, and (c) a method including the steps of dissolving iron alkoxide, nickel acetylacetonate and zinc acetylacetonate in an organic solvent, adding water to the resultant solution to hydrolyze the alkoxide and acetylacetonate, and calcining the resultant gel to produce a Ni--Zn ferrite powder.
However, these methods have the following respective disadvantages awaiting a solution. For example, the powder-mixing method has the disadvantage that it is impossible to disperse all the ingredients homogeneously on atomic levels because the starting materials are powders of oxides and/or carbonates. Thus, it is necessary to calcine the mixture of the starting materials at temperatures of 700.degree. C. and above to make it spinel. In addition, it is impossible to avoid local deviation of the composition.
Although each starting material may be produced by calcination of fine precipitates prepared by the wet process, the resultant powder is large in particle size and low in activity because of aggregation of particles even if the precipitate is fine particles. The use of such ferrite powder results in the increase in sintering temperature. Further, it is difficult to disperse all the ingredients homogeneously on atomic levels. In addition, it is necessary to make the aggregated particles into finely ground particles, but it adulterates the ferrite powder because of wear of media and inner walls of a mill.
To solve such problems, it has been proposed to incorporate an auxiliary sintering agent into the ferrite powder. This contributes to lower the sintering temperature, but causes lowering of magnetic properties of the ferrite products.
On the other hand, method (b) occasionally produces oxalate of the respective metal ions along with a desired molecular compound because of difference in optimum pH for production of oxalate between the ferrite elements, causing formation of a mixed precipitate, which in turn causes production of heterogeneous ferrite powder. Since the compounds in the mixed precipitate differ from one another in solubility, they differ from one another in elution during washing, resulting in deviation from the composition to be produced.
The above oxalate compounds are then calcined to form them into a spinel compound. This reaction mechanism begins with decomposition of oxalate compounds into carbonates and oxides by release of CO.sub.2 or CO from the carboxyl group, which then react with one another in the solid phase. Thus, it is necessary to calcine the compounds at a high temperature, causing adulteration due to pulverization of the ferrite powder. In addition, this oxalate method requires liquid waste treatment including aeration for lowering of B.O.D. or neutralization of acidic solutions.
The method (c) makes it possible to produce magnetic oxide powder which is low in calcination temperature but high in surface activity and purity. However, this method involves the problem that the starting materials to be used are too expensive to mass-produce the magnetic oxide powder.